A leading researcher has called for live-bird markets in H7N9-hit areas of the mainland to be closed and poultry culled to block the deadly virus' spread, as a study confirmed wet markets to be the "smoking gun" in the spread of the disease.

A report in The Lancet medical journal yesterday provided the first evidence that the new flu virus in human infections is 99.4 per cent related genetically to that found in live chickens.

Jiangxi province yesterday became the eighth region, and the first bordering Guangdong, to report a confirmed case of the flu.

The Jiangxi provincial government said it would strengthen the monitoring of poultry but insisted there was no need to close markets or cull birds.

Some mainland cities, however, have already shut markets at the first sign of an infection.

University of Hong Kong microbiologist Yuen Kwok-yung said: "The [Lancet] research is the first direct proof of poultry markets being the smoking gun, the chicken virus being the bullet, with the patient being the victim.

"The difference between the chicken and human virus is so small that the chicken virus must have just recently jumped into humans," said Yuen, who collaborated on the study with Zhejiang University.

Though the virus' ability to spread from birds to humans is still limited, the study said it displayed a high ability to spread between chickens - with 20 per cent of chickens in linked markets found to be infected by H7N9.

Click on each balloon for more information on individual patients infected: blue, patients infected with the H7N9 virus under treatment; red, those infected with H7N9 who have died; yellow, those who have fully recovered; and pink, those infected other types of the Influenza A virus, including H1N1.

Further adaptation could lead to less symptomatic infection and more efficient person-to-person transmission, the study said. No transmission among humans was found in 303 household or workplace contacts and 82 health-care workers who had contact with the four patients studied in the research.

Yuen said aggressive intervention, such as temporary closure of live bird markets, culling and eventually a centralised way of slaughtering, should be considered at this stage to block further animal-to-human transmission in live poultry markets.

Professor Lu Jiahai, an epidemiologist at Sun Yat-sen University's school of public health in Guangzhou, warned that Jiangxi could be a stepping stone for the virus to move from the Yangtze River Delta to the Pearl River Delta.

"With the change of temperature and humidity in various regions, the Yangtze River Delta may become less comfortable for the virus, while other regions such as Guangdong may become more attractive," he said.

Elsewhere on the mainland there were three new cases yesterday - two in Zhejiang and one patient in Henan described as being critical. A total of 112 infections have been reported so far, with 23 deaths.

Taiwan heightened surveillance of travellers from the mainland yesterday after authorities confirmed the island's first case of H7N9 bird flu.

This article appeared in the South China Morning Post print edition as: